This invention relates to a color separating circuit for distinguishing a given color from other colors on an original.
Consider first an original having a white field on which picture data have been recorded in various colors. It is frequently required to distinguish picture data, for instance, in red from the other picture data and to record or display the picture data thus distinguished. In such a case, a color separating circuit is employed in order to distinguish and separate a given color from other colors.
FIG. 1 illustrates one example of a reading device with such a color separating circuit. An original 2 is placed on a transparent flat glass plate, namely, a platen 1 in such a manner that the surface to be read is faced downward. A pair of fluorescent lamps 3 and 4 are arranged below the platen 1 in such a manner that they extend in the main scanning direction (perpendicular from the plane of the sheet). Therefore, the surface of the original 2 being read is exposed to a narrow band of light.
A light beam 5 reflected from the orignal 2 is split into two light beams 5.sub.1 and 5.sub.2 by a halfmirror 6. The light beam 5.sub.1, which has passed through the half-mirror 6, forms the image on a first image sensor 8 with the aid of a first lens 7. On the other hand, the light beam 5.sub.2, which has been reflected by the half-mirror 6, is converged by a lens 9 and is subjected to wavelength selection by a filter 11, thus forming the image on a second image sensor 12. When the reading device distinguishes a color, for example, red from the other colors, a cyan filter, the color of which is complementary with red, is employed as the filter 11.
FIG. 2 indicates the levels of picture signals provided by the two image sensors of the reading device. More specifically, the waveform (a) of FIG. 2 shows the picture signal provided by the image sensor 8 in FIG. 1, while the waveform (b) of FIG. 2 shows the picture signal provided by the image sensor 12. These picture signals, after being digitized with threshold levels l.sub.1 and l.sub.2 indicated by the dotted lines, are supplied to a color separating circuit.
FIG. 3 shows a conventional color separating circuit. A signal 16 which is obtained by digitizing the output picture signal of the first image sensor 8 (hereinafter referred to as "a white binary signal 16", when applicable) is applied to a first input terminal 15 of the color separating circuit. Similarly, a signal 18 is obtained by digitizing the output picture signal of the second image sensor 12 (hereinafter referred to as "a complementary color binary signal 18", when applicable) to a second input terminal 17. The white binary signal 16 is applied unaltered to a first output terminal 19 and to one input terminal of a 2-input NAND circuit 21. The complementary binary signal 18 is applied through an inverter 22 to the other input terminal of the NAND circuit 21. The output of the NAND circuit 21 is applied to a second output terminal 23. The circuit is designed so that, when a signal "0" is provided at the first output terminal 19, the input picture data is judged to be black, and that, when a signal "0" is provided at the second output terminal 23, the input picture data is judged to be red.
When white picture data is applied to the color separating circuit, signals "1" are provided at the first and second output terminals 19 and 23, respectively. That is, in this case, the circuit recognizes the picture data to be white. When red picture data is applied to the circuit, a signal "1" is provided at the first output terminal 19, while a signal "0" is provided at the second output terminal 23. In this case, the circuit determines the data to be red. When blue, green or black picture data is inputted to the color separating circuit, a signal "0" is provided at the first output terminal 19 while a signal "1" is provided at the second output terminal 23. In this case, the circuit recognizes the data to be black. Thus, the circuit can satisfactorily separate red from other colors, such as white, blue, green and black.
However, the color separating circuit sometimes recognizes a color which is clearly different from the particular color to be distinguished from other colors. For instance, when yellow picture data as shown in FIG. 2 is inputted to the above-described color separating circuit for red separation, the signals "1" and "0" are provided at the first and second output terminals 19 and 23 in FIG. 1, respectively. This is the same result as that in the case where red picture data is inputted. Of course, the yellow picture data may be recognized to be white by decreasing the threshold level l.sub.2 in waveform (b) of FIG. 2. However, when the threshold level l.sub.2 is lowered, light red may be judged to be white. This is not preferable, because the red picture data which should be printed in red ink is therefore not printed.